Resistance spot welding typically comprises pressing round weld face surfaces of two opposing, high conductivity, copper electrodes against opposite sides of two or three overlapping metal sheets (sometimes called a “stackup”), and passing an electric current for a period of several milliseconds to a few hundreds of milliseconds between the electrodes through the sheets to form a weld nugget at the sheet-to-sheet interface, called the faying interface.
Resistance spot welding of aluminum workpieces (typically aluminum-based alloys containing 85% or more by weight aluminum) at high volume is considered to be very difficult within the automotive industry because of several issues. The workpieces are often rolled aluminum alloy sheet materials, but may also be extrusions or castings of suitable complementary shape for spot welding. While complementary-shaped aluminum-based alloy body panels may be placed together and joined by a series of suitably located spot welds, the aluminum workpieces may be of equal or different thicknesses, of the same or different aluminum alloys, coated on the surface, and may have adhesives or sealants applied along weld flanges. There may be small gaps between the assembled panels and one or both of the opposing welding electrodes may be positioned at an angle slightly different from its intended welding position.
One of the major issues is the presence of a tough, adherent, non-conducting oxide film on the aluminum substrate surface. This oxide film can cause excessive overheating at both electrode/sheet interfaces as well as the sheet-to-sheet faying interface. Typical solutions to the problem of electrode/sheet interface overheating include the use of electrodes designed with large, flat weld faces that reduce the current density and, thus, heating at these locations. The use of large, flat electrodes produces undesired consequences for manufacturing. These types of electrodes are 1) sensitive to gaps between workpieces, 2) sensitive to electrode orientation, i.e., being off-angle or off-normal with respect to the workpiece surface, and 3) require large flanges on the workpieces to accommodate the large electrode body diameter and weld face on the electrode.
Several electrode designs and dressing processes that address these issues may be found in patents and patent applications, including one or both of the inventors herein and owned by the assignee of this invention: U.S. Pat. No. 6,861,609 (Mar. 1, 2005) and U.S. patent applications published as 20100258536, 20090302009, 20090255908, 20090127232, 20080078749. The problem of the oxide film and resultant electrode/sheet overheating has been addressed by placing geometric features, such as a micro texture or a series of ridges and grooves on the weld face that, under weld load, penetrate the oxide layer on aluminum to lower contact resistance and heat generation at that interface. The reduction in electrode/sheet heating has two direct benefits. First, it allows a smaller electrode with less thermal mass to be used, which decreases flange requirements. Second, it allows a sharper electrode weld face curvature to be used that better concentrates welding current. This makes the welding process much less sensitive to both the electrode orientation on the workpiece, i.e., the electrode being off-angle with respect to the workpiece, and the presence of gaps between workpiece surfaces.
Despite these very significant improvements in process performance attained by solving the issue of high contact resistance at the electrode/sheet interfaces, issues remain to be solved before the aluminum spot welding process is considered sufficiently robust for high volume manufacturing. Many of these issues are related to the presence of surface oxide films at the sheet-to-sheet or faying interface, which is unaffected by modifications to the electrode weld face. These issues are related, in part, to the nature of the direct current welding process typically used in automotive aluminum welding which is termed Medium Frequency Direct Current or MFDC. This process uses an inverter type weld control that receives a three phase, 60 Hz alternating current potential at 480 volts rms (in the United States) and provides a single phase square wave of higher voltage, about 650 volts, to the MFDC transformer at a frequency of approximately 1000 Hz. The transformer reduces the high voltage waveform supplied by the weld control to a much lower welding voltage (for example 13 volts at a 50:1 transformer turn ratio) at much higher current. The low voltage, square wave output at the transformer is then rectified with high current capacity diodes to provide DC current for delivery to the welding electrodes and stackup of workpieces. During setup for producing many like welds on a series of workpieces, a suitable weld current and weld time are predetermined. The MFDC weld controller is then programmed to deliver a nearly constant current (e.g., twenty-five to thirty kilo amperes) to the welding electrodes pressed against a workpiece over a weld cycle of about 250 to 300 milliseconds. In any DC process, and particularly one where current flow is programmed to be nearly constant, as with MFDC, one electrode (positive) runs considerably hotter than the other electrode (negative) when in contact with aluminum substrates. This temperature bias of the electrodes can affect weld nugget formation and growth, especially for stickups of sheet workpieces that are asymmetrical with respect to both thickness, i.e., high thick/thin ratios, and material, e.g., welding Aluminum Alloy 5754-O to Aluminum Alloy 6111-T4 sheet. AA5754-O composition limits are 2.6-3.6% Mg, <0.4% Si, <0.5% Mn, <0.4% Fe, and <0.1% Cu (balance substantially all aluminum), while age hardenable AA-6111-T4 composition limits are 0.5-1.0 Mg, 0.6-1.1% Si, 0.1-0.45% Mn, <0.4% Fe, and 0.5-0.9% Cu. This can result in stackups welding better in one orientation relative to electrode polarity than the other including producing larger welds or greater weld penetration in one orientation than the other, which in production operations would not be ideal. In addition, the hotter running positive electrode is more prone to wear and, thus, can shorten electrode life by requiring more frequent dressing.
In addition to the polarity effects, the standard constant current weld schedules that have been used in production applications of aluminum spot welding can produce other undesired issues. These schedules are based on the application of a constant current, e.g., 27 kA, over a set time, e.g., 200 milli-seconds (ms), at a constant force of the electrodes against the workpiece surfaces. The issues that have been discovered include excessive electrode wear, sensitivity to weld spacing for heavy gauge stackups, inconsistent size and quality of the first weld, weld microstructures that lead to undesirable weld fracture modes, and weld shapes that lead to premature expulsion and poorer weld quality. The undesirable fracture modes, which occur in peel or tensile loading, include weld fracturing along the faying interface or fracturing around the weld nugget perimeter and, thus, not forming a button that pulls completely through the sheet thickness. Finally, the standard constant current weld schedules are less robust in the presence of sealers or adhesives. With adhesives or sealers present, these schedules tend to result in nuggets with more defects that tend to fracture in undesirable modes especially when subject to peel loading.
There remains a need for improved practices for resistance spot welding of aluminum alloy sheet metal workpieces and other workpiece shapes.